The electronics industry utilizes dielectric materials as insulating layers between circuits and components of integrated circuits and associated electronic devices. Line dimensions are being reduced in order to increase speed and storage capability of microelectronic devices (e.g., computer chips). Microchip dimensions have undergone a significant decrease even in the past decade such that line widths previously greater than 1 micron are being decreased to 0.18 microns, with future plans on the drawing boards of at least as low as 0.07 microns. The time delay expression T= 1/2  RCL2, where T is the delay time, R is the resistance of the conductive line, C is the capacitance of the dielectric layer, and L is the wire length, is often used to define the effects that changes in dimensions and materials can have on the propagation of signals in a circuit. The capacitance can be expressed as C=k0k (S/d), where k0 is the vacuum permitivity or dielectric constant (equal to 1.0), k is the dielectric constant for the thin film, S is the electrode surface area and d is the film thickness. Thus, a decrease in k will result in a proportional reduction in C and consequently a reduction in delay time. Further, as the line dimensions decrease, better insulating materials with lower dielectric constants are also needed to prevent signal crossover (aka crosstalk) between the chip components, which can have a negative effect on performance.
Historically, silica with a dielectric constant (k) of 4.2–4.5 has been employed as the interlayer dielectric (ILD). However at line dimensions of 0.25 microns and less, silica may no longer be acceptable, and it has been extensively replaced by other materials, such as fluorinated silica glass (FSG) wherein k is about 3.6. The addition of fluorine to silica specifically aimed at reducing the k value from that of undoped silica has been studied for the past few years (see, e.g., U.S. Pat. Nos. 5,571,576, 5,661,093, 5,700,736, 5,703,404, 5,827,785 and 5,872,065). The high electronegativity of fluorine results in a very non-polarizable species, which reduces the dielectric constant. Fluorinated silica has gained acceptance in the industry and is being used for the current generation of ICs.
While fluorinated silica materials have the requisite thermal and mechanical stability to withstand very high temperatures (up to 500° C.), the materials' properties (e.g., low water sorption, mechanical properties) are susceptible to being compromised when large amounts of fluorine are incorporated into the material. Fluorinated organic materials, such as poly(tetrafluoroethylene) despite having very low k values down to 2.0 or less, have not shown sufficient stability to the temperatures experienced during subsequent processing steps involved in the manufacture of an integrated circuit. Organic polymers in general do not possess sufficient mechanical strength for processing under current conditions. As well, fluorocarbon polymers can have other drawbacks such as poor adhesion, potential reaction with metals at high temperature, and poor rigidity at high temperature in some cases. In order to achieve the desired property characteristics and low dielectric constant values, silica based dielectric films which incorporate both organic dopants and inorganic fluorine species may provide for films with k values lower than FSG, and better thermal and mechanical properties than organosilica glass (OSG) materials, while maintaining the requisite properties to function as an interlayer/intermetal material in IC manufacturing.
More recently, OSG is being sought as the replacement for FSG. OSG materials are being touted as the future interlayer/intermetal dielectric of choice produced by CVD techniques. Numerous patents have been issued covering the use of various organosilanes for the production of thin films with k values of about 2.7–3.2 (see, e.g., U.S. Pat. Nos. 5,989,998, 6,054,379, 6,072,227, 6,147,009 and U.S. Pat. No. 6,159,871, and WO 99/41423). OSG thin film dielectric materials are being commercialized and/or advertised by several leading OEMs for future ICs due to their inherently lower k (<3.2) relative to FSG. However, the reduction in k must be balanced against detrimental effects that organic species typically have, which include reduced mechanical properties, thermal stability and chemical resistance. Studies have indicated that the preferred materials properties for OSG limit the dielectric constant to the range of 2.8–3.2 with modulus/hardness values in the, range of 9–11/1.2–1.4 GPa (see Lee et al., 198th Meeting of The Electrochemical Society, October 2000, Section H-1, Abstract No. 531; and Golden et al., MICRO, p. 31, February 2001).
Some recent literature and patents have proposed the use of carbon-doped FSG materials. Most of these examples exclusively utilize fluorocarbon materials as precursors in combination with a silicon precursor source, and incorporate fluorocarbon moieties into a silica or FSG framework. For example, Shirafuji et al. plasma copolymerized hexamethyidisiloxane with octafluorobutene (Plasmas and Polymers, 4(1) (57–75) March 1999 or tetrafluoroethylene (38 Jpn. J. Appl. Phys. 4520–26 (1999)) to produce fluorocarbon/SiO composite films in which k increased from 2.0 to 3.3 with decreasing fluorocarbon content. Yun et al. (341 (1,2) Thin Solid Films 109–11 (1999)) discuss the effects of fluorocarbon addition to SiOF films produced in a helicon plasma reactor using triethoxyfluorosilane and O2.
Another example of the specific inclusion of fluorocarbon moieties in silica is the work of Kim et al., 1998 IEEE International Conference On Conduction and Breakdown in Solid Dielectrics 229–32 (1998), describing the ability of fluorocarbon addition to reduce the k of the material substantially from that of silica. The work of Kim et al. appears to be aimed specifically at incorporating fluorocarbon moieties through the use of CF4 in a 2% silane/N2 plasma to produce films containing silicon, oxygen, carbon, fluorine, and N, where they were able to identify Si—C, Si—N, Si—O, and C—F functionalities. They also found that there was a depth profile to their compositions, whereby the surface was higher in oxygen than the bulk.
U.S. Pat. No. 5,800,877 to Maeda et al. describes the use of a mixture of organosilane precursors having an Si—F bond and organosilane precursors without an Si—F bond, with ozone or oxygen, in a thermal process to produce a fluorine-containing silicon oxide film. The claims of this patent cover the production of a fluorine-containing silicon oxide via thermal process with an oxygen and/or nitrogen plasma post-treatment. The patent does not describe the incorporation of alkyl groups or carbon into the film.
In a paper by Hasegawa et al. (37 Jpn. J. Appl. Phys. 4904–09 (1998)), enhanced water resistance of fluorinated silica was the motivation for deposition using mixtures of silane, oxygen, CF4 and ammonia in a plasma-enhanced CVD system. The deposited films were found to contain a significant amount of Si—N and C—F bonds, as interpreted by XPS spectra. Enhancing the water resistance via incorporation of Si—N will negatively impact the k value.
In similar works by the same group noted above, Lubguban et al. (337 Thin Solid Films 67–70 (1999), 606 Materials Research Society Symposium Proceedings 57 (2000), and 87(8) Journal of Applied Physics 3715–22 (2000)) discuss the introduction of carbon into fluorosilicate glass by PE-CVD to enhance water resistivity. The materials were synthesized from silane or TEOS, oxygen, methane and perfluoromethane, and were studied for composition, thermal stability, and electrical properties. Lubguban et al. suggest that the incorporation of both carbon and fluorine into a SiO2 network reduces the dielectric constant. Increases in the amount of methane introduced to the deposition chamber during reaction resulted in increased carbon and fluorine in the final material, said to be caused by a significant contribution by C—F functionalities. As described in their papers, the presence of C—F and C—H species will promote resistance to water sorption and help to reduce dielectric constant.
In a Japanese patent by Fujitsu (JP10150036A2), organosilicon materials deposited by a spin coat method had a post-deposition treatment with F2 or NF3 in a plasma reactor to increase heat resistance, reduce water sorption, and increase material reliability of the film through the formation of fluorocarbon species in the film. Other Fujitsu patents (JP 8321499A2 and JP 11111712A2) also discuss the formation of silica films with incorporated fluorocarbon species by plasma CVD using silicon-based precursors containing fluorocarbon groups.
Uchida et al. disclose fluorinated organic-silica films for improved moisture tolerance. See, e.g., 98(3) Electrochem. Soc. 163–8 (1998), 37 Jpn. J. Appl. Phys. 6369–73 (1998), 38 Jpn. J. Appl. Phys. 2368–72 (1999) and JP 11111714A. In these papers, the authors indicate that the properties of FSG and OSG may be complementary, such that a material that has both functionalities may take advantage of their strengths, although little supporting data is given. The authors attempt to show this asserted advantage by describing a process in which an organosilicon material was deposited in a thermal process from a mixture of tertiary methylamine (TMA), tetraisocyanate-silane (TICS), dimethyidiisocyanate-silane (DMSIC) and preferably dimethylethylamine (DMA), to produce H and OH—free silica films. This deposited film was post-treated with HF gas in a thermal process to replace isocyanate species with fluorine, and produced films with a lower dielectric constant and better moisture tolerance. The films produced, however, included C—Si and C—F functionalities. Also, as is typical in diffusional-based processes such as chemical post-treatments, there resulted a compositional gradient induced through the depth of the film. It is inherently difficult to control the amount and uniformity of the chemical modification through the film in this manner.
U.S. Pat. No. 6,077,574 to Usami discloses processes for forming a plasma CVD silicon oxide dielectric film doped with specified amounts of fluorine and carbon, wherein the film is deposited from a mixture of feed gases, which can control the fluorine concentration and the carbon concentration independently of each other. The atomic ratio ([carbon]/[fluorine]) in these films is less than or equal to 0.25, based on the disclosed concentration ranges of 4.0×1021 fluorine atoms/cc to 1.0×1022 fluorine atoms/cc, and 3.0×1019 carbon atoms/cc to 1.0×1021 carbon atoms/cc. No data are presented regarding the functionalities formed within the film. Given that the density of silica is about 2.2 grams/cc, which is equivalent to 6.6×1022 atoms/cc, the concentration of fluorine and carbon can be estimated to be about 6–15 atomic % fluorine and about 0.05 to 1.5 atomic % carbon.
U.S. Pat. No. 6,410,463 to Matsuki discloses a method for forming a low dielectric constant film that increases the residence time of the reaction gas in the reactor to at least 100 msec. Matsuki does not disclose the use of an oxygen-providing gas (e.g., O2) in the production of the film.
Despite the foregoing developments, there have not been any examples in the prior art that successfully combine the desired mechanical, dielectric, thermal and oxidative stability properties that are paramount for integrating low k dielectric materials in integrated circuits.
All references cited herein are incorporated herein by reference in their entireties.